1. Field of the Invention
The present invention relates generally to a diesel engine fuel injector system, and more particularly to an electronically controlled spill port for a fuel injector.
2. Description of the Background Art
Fuel injectors are devices used to meter out precise volumes of fuel into a cylinder of an engine. They are commonly used for purposes of precise fuel control, increased fuel economy, and emissions reduction. By accurately controlling the rate and volume of injected fuel and the time in the engine cycle when the fuel is injected, a fuel injector can be used to achieve the above goals.
The onset, rate, and duration of fuel injected into a diesel engine has been proven to affect BsNOx and BsPt emissions levels, as well as affecting BsFC. BsNOx is a measure of Brake specific Nitrogen Oxide emissions, such as NO and NO2 pollutants. BsPt is a measure of Brake specific lead (Pt) emissions, another pollutant generated by an engine. BsFC is the Brake specific Fuel Consumption, which is a measure of fuel rate in pounds per hour divide by power output (lb/hp-hr).
A high cam velocity and high hydraulic flow nozzle (short injection durations) can provide minimum fuel consumption. However, with this aggressive injection system, injection timing cannot be retarded enough to meet U.S. 1998 BsNOx standards without misfire and a rapid increase in BsPt emissions levels. The reason for this is the high fuel injection rate associated with a high velocity cam and high hydraulic flow nozzle, as shown in the chart of FIG. 1A. It has been well documented that the fuel injection rate significantly impacts BsNOx emissions levels, especially the injection rate during the first 5-10 engine degrees of injection. As the injection rate increases, the BsNOx emissions levels also increase.
The effort to reduce emissions through more precise control of fuel injection has led to several related art approaches. One simple method uses a slower velocity cam and a lower hydraulic flow nozzle, as shown in the chart of FIG. 1B. This allows low BsNOx and BsPt emissions levels without retarding injection timing so much as to cause misfire. This system will, however, increase injection duration and will therefore impact highway fuel consumption.
Another more complicated method for allowing lower BsNOx emissions levels to be obtained with any injection system is to inject a small quantity of xe2x80x9cpilotxe2x80x9d fuel before the main injection (i.e., pilot injection). Pilot injection is depicted in the chart of FIG. 1C. This small pilot quantity of fuel does not reduce the rate of injection but will allow more retarded main injection timings without misfire, thus allowing lower BsNOx emission levels without a rapid increase in BsPt emissions levels. However, as main injection timing is retarded to control BsNOx, the BsPt solids emissions levels will gradually increase due to a later occurring end of injection. It is therefore possible that a system optimized for minimum fuel consumption (very high rate of injection) would require such retarded timings to meet U.S. 1998 BsNOx emissions standards that the BsPt emissions levels may exceed the 1998 targets, even if pilot injection is utilized. At any rate, very retarded injection timings can cause several other problems such as poor fuel consumption, high heat rejection, excessive turbo wheel speed and the requirement of a large timing range designed into the cam profile.
A further refinement of the precise control of fuel injection is the use of a spill valve. A spill valve allows the spilling of fuel from the injector during the injection cycle. Spill valves are used because fuel injectors are mechanical devices, driven off of a camshaft. A cylinder within the injector is driven by the cam, and provides a fuel volume and pressure as dictated by the timing and aggressiveness of the cam. The operation of the injector cylinder is mechanically fixed by the cam, and cannot be varied during operation of the engine. In order to more precisely control the fuel injection, such as by electronic means, a spill valve is used to discard some of the pressurized fuel. The spill valve can be opened at any time in the injection cycle (i.e., when the injector cylinder is pressurizing the fuel) to spill excess or unneeded fuel.
One approach is to have a spill valve designed into the plunger/barrel assembly of an injector. This approach is currently utilized by Navistar with the HEUI (PRIME) system and is illustrated in FIGS. 2A and 2B. The spill valve is fixed in location and spills a portion of the high pressure fuel during the initial part of an injection stroke, as can be seen in FIG. 2A. However, the HEUI (PRIME) system is a fixed spill valve which cannot vary the injection opening timing and flow rate in order to minimize emissions levels for a full range of engine loads.
Another approach in the related art is given in Cananagh, U.S. Pat. No. 5,333,588. Cananagh discloses a fuel injector having an electromagnetically controlled spill valve, and may include two such spill valves. Cananagh proposes two spill ports in order to cope with large displacements of fuel per injector plunger stroke. The purpose of dual spill valves in Cananagh is to increase the flow area through which fuel can escape from the injector pumping chamber. In addition, Cananagh discloses a non-synchronized opening of the spill valves where one valve can be energized slightly before the other to provide variation of the initial rate of delivery of fuel. This is apparently done to forestall a premature high fuel pressure at the inlet of the injection nozzle. If the fuel pressure exceeds a nozzle opening pressure, the injector nozzle may open prematurely. Apparently the goal of Cananagh in early closing of one spill valve is to delay the opening of the injector nozzle by forestalling a high fuel pressure.
What is needed therefore is a spill valve system wherein more than one fuel injection rate can be obtained in order to rate shape the fuel injection profile.
A diesel engine fuel injection system is provided according to a first aspect of the invention. The diesel engine fuel injection system comprises a fuel injector for injecting fuel into a corresponding engine cylinder, each fuel injector having a pump chamber, a fuel injecting plunger for reciprocating within the pump chamber, a supply line connected to the pump chamber for receiving fuel, and a discharge nozzle connected to the pump chamber and to the corresponding cylinder for injecting fuel into the corresponding cylinder, a cam shaft having a respective cam operably connected to the plunger of the corresponding fuel injector so that rotation of the cam causes reciprocation of the plunger and movement of fuel from the supply line through the chamber to the corresponding cylinder, and a spill valve positioned between the chamber and the nozzle for controlling a rate of fuel injection to the corresponding cylinder, the spill valve having a first position providing a maximum fuel injection rate, a second position providing a substantially zero fuel injection rate, and at least one intermediate position providing an intermediate fuel injection rate between the maximum fuel injection rate and the zero fuel injection rate.
A diesel engine fuel injection system is provided according to a second aspect of the invention. The diesel engine fuel injection system comprises a fuel injector for injecting fuel into a corresponding engine cylinder, each fuel injector having a pump chamber, a fuel injecting plunger for reciprocating within the pump chamber, a supply line connected to the pump chamber for receiving fuel, and a discharge nozzle connected to the pump chamber and to the corresponding cylinder for injecting fuel into the corresponding cylinder, a cam shaft having a respective cam operably connected to the plunger of the corresponding fuel injector so that rotation of the cam causes reciprocation of the plunger and movement of fuel from the supply line through the chamber to said corresponding cylinder, and at least two spill valves positioned between the chamber and the nozzle for controlling a rate of fuel injection to the corresponding cylinder, providing a maximum fuel injection rate when both of the at least two spill valves are open, providing a substantially zero fuel injection rate when both of the at least two spill valves are closed, and providing an intermediate fuel injection rate between the maximum fuel injection rate and the zero fuel injection rate when one of the at least two spill valves are open.
A method for rate shaping a fuel injecction profile in a diesel engine is provided according to a third aspect of the invention. The method comprises the steps of pressurizing fuel fed to a fuel injector nozzle, partially opening a spill valve communicating with the fuel injector nozzle, so that the fuel injector injects fuel into a corresponding engine cylinder at a first fuel injection rate for a predetermined first period of time during an engine fuel injection cycle, and fully opening the spill valve so that the fuel injector injects fuel into the corresponding engine cylinder at a second fuel injection rate for a remainder of the engine fuel injection cycle, wherein the first injection rate and the second injection rate shape a fuel flow rate of injected fuel.
The above and other objects, features and advantages of the present invention will be further understood from the following description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings.